Use of cold climate heat pumps has been primarily limited by low capacity in very cold climates, and by low discharge temperatures resulting in “cold blow”, a condition that occurs when the supply air temperature is warm enough to heat a room, but feels cold impinging on a person. The performance gap of some such systems in cold climates stems from a reduction in volumetric flow due to significantly reduced compressor suction density as well as reduction in compressor isentropic efficiency at higher pressure ratios.
Some known systems employ two to three scroll compressors to meet capacity and volumetric flow requirements. However, scroll compressors are fundamentally limited by their fixed volume ratio (for those without a discharge valve), or by the limited space for a single discharge port and valve (for those using a discharge valve). Further, such systems may require supplemental heating (e.g., electric or natural gas supplements) to achieve a desired thermal comfort in cold climates. Accordingly, it is desirable to provide a cold climate heat pump system with increased capacity and COP (coefficient of performance) at extremely low ambient temperatures.